Whats the best format to turn a render into a poster?
First determine what size (in cm or inches) you want the poster to be.

Then decide on the quality of the print: find out how many DPIs you need for the purposes you are after. 300 DPI is the common reslolution for prints, but it might vary.

Find an online calculator to to determine the dimensions in pixels for your image you need to render.

Set your scene in blender to render using those dimensions.

Save to a Tiff or PNG file.

The biggest challenge for such a project is that you can preserve the colors and quality you see on your monitor to those possible on printed media. For that it's better to consult with the printing service you'll use,and find out from them what kind of image format they prefer to work with, and how to deal with color conversion from RBG to CMYK.

If possible print a small test.